1. Field of the Invention
The present invention relates to a compact loop antenna.
2. Description of the Related Art
Loop antennas have conventionally been used as basic antenna. A loop antenna is simply structured, easily made compact and deployed at low cost, thus being used for conventional inductive read/write apparatus.
In recent years, the inductive read/write apparatuses (e.g., noncontact type cards) have been put into use, such as noncontact type cards conforming to ISO standards, i.e., ISO14443, ISO15693, et cetera. Among them, communication systems conforming to ISO14443 are adopted as the noncontact type cards performing at a communication range up to 10 cm in many cases. Meanwhile, the ISO15693 standard provides for a communication range up to approximately 70 cm and therefore many apparatus utilize it as an RF-ID (Radio Frequency Identification). When mounting a loop antenna onto an inductive read/write apparatus, the general practice is to form the loop antenna by using a printed circuit board for applying an etching of a spiral pattern with a plurality of turns of the spiral.
For example, FIGS. 1A and 1B show an example of a conventional configuration of a loop antenna.
FIG. 1A shows a top view of the loop antenna including a printed circuit board 30 on which a loop pattern 29 is formed, a loop pattern 29 formed by an etching process, et cetera, on the printed circuit board 30, a start point terminal 31 and an end point terminal 32 for supplying power for transmitting a signal and outputting a received signal.
Incidentally, the example configuration shown by FIG. 1A shows the loop pattern 29 in the case of forming a loop with two turns of the spiral.
FIG. 1B shows a cross section of the loop antenna shown by FIG. 1A. As shown by FIG. 1B, a conventional loop antenna is formed by a plurality of loops of different sizes in order to form a spiral loop pattern on one board.
A patent document, laid-open Japanese patent application publication No. 54-008441, has disclosed a loop antenna in a combination of a plurality of loop antenna elements, with different loop radii around the same center, which are placed on the same plane. And a patent document, laid-open Japanese patent application publication No. 11-272826, has disclosed a loop antenna, applying to an IC card, et cetera, which is configured so that the conjunction of a plurality of spirals is non-dense in order to improve the efficiency of an antenna and increase a communicable distance in a comprisal for receiving power supply by the antenna.
In a spiral loop antenna formed by a plurality of spirals such as the loop antenna shown by FIGS. 1A and 1B, an antenna area (i.e., an area through which flux linkage passes within a loop formed by a loop pattern of the loop antenna) and the power which can be supplied to the antenna are actually limited by design constraints such as the size of a transmitter or receiver on which the loop antenna is mounted, or a battery capacity. The allowed antenna area and supplied power, et cetera, make it difficult to increase a field intensity in the direction perpendicular to the antenna face, hence it is very hard to increase a read/write distance (i.e., communication range).
Here, it is conceivable to etch a loop pattern by increasing the number of spiral turns further to four or five on a single board in the antenna shown by FIGS. 1A and 1B, but the field intensity increases only in the direction horizontal to the antenna face and not in the perpendicular direction (NB the field follows the right-hand screw rule and is generated in the direction of the right rotation with respect to the direction of current flowing in the pattern. If there is a plurality of patterns which are lined up in the horizontal direction and therefore the field does not increase in the perpendicular direction). Although there is a method of increasing transmission power to strengthen a radio wave, the size of power supply unit becomes large, generating more heat and increasing cost. Such is the problem.
For all this, increasingly in demand recently is a technique to extend a communication range with a small antenna area and low power.